The purpose of this study is to investigate the nature of energy metabolism in the neonate through quantitative animal studies and correlative clinical approaches. Animal Studies: A subhuman primate model (the neonatal and infant baboon) is being used for quantitative investigations of energy metabolism. Appropriately placed arterial and venous catheters for sampling of arteriovenous metabolite differences and the radioactive microsphere technique for measuring organ blood flows have allowed quantitation of cerebral, splanchnic and renal substrate uptake production. Clinical Studies: Arterial and venous substrate levels in high risk infants have been monitored prospectively. Correlations with clinical condition are being made. BIBLIOGRAPHIC REFERENCES: Levitsky, L.L., Paton, J.B. and Fisher, D.E. "Splanchnic production and extraction of gluconeogenic precursors in the fasting baboon neonate," presented, Soc. Pediatr. Res., 1975. Levitsky, L.L., Paton, J.B., Fisher, D.E. and DeLannoy, C.W., "Blood levels of gluconeogenic precursors and renal gluconeogenesis in the fasting baboon neonate." accepted for presentation, Soc. Pediatr., Res., 1976.